Figure 2-60. Chromatic Aberration

TM 9-258

color. Similarly, spherical aberration is greater for blue

formed by a positive lens will show compression, while

rays than for red rays because the focal length. of a lens

the marginal parts of a blue virtual image viewed through

is shorter for blue than for red rays. This is due to red

a positive lens will be more distended than the red rays.

rays being refracted the lesser of the two. The marginal

parts of a blue real image

Figure 2-60. Chromatic Aberration

b. The aforementioned chromatic aberration, to

prevent trouble in optical instruments, must be corrected

by spacing between lenses and by adjusting curvatures.

A, figure 2-61, illustrates the diminishing to such effects

by equalizing the deviation at the two surfaces. Like

spherical aberration, chromatic aberration is corrected by

making a compound lens of two separate lenses; one

positive (convergent) and one negative (divergent). The

positive lens is made of crown glass while the negative

lens is made of flint glass (B, fig 2-61). Crown glass is

more strongly convergent for blue rays than for red, while

flint glass is more strongly divergent for blue rays than for

red. The high color dispersion of the flint glass negative

lens is sufficient to compensate for the lower color

dispersion of the crown glass positive lens without

entirely neutralizing its refractive power. A compound

lens so designed is said to be achromatic. Construction

of a lens of this type is described in paragraph 4-2b.

Figure 2-61. Correction of chromatic aberration of lens.

2-45